a similar class of the population at sea-level.
It is only when a sufficient number of cases occur in succession to
raise the virulence of the pneumococcus in this curious manner that an
epidemic with high fatality develops.

That this increase in virulence in the organism does occur was clearly
demonstrated by a bacteriologist friend of mine, who succeeded in
securing some of the sputum from a fatal case in the famous Tonopah
epidemic of some years ago, an epidemic so fatal that it was locally
known as the "Black Death." Upon injecting cultures from this sputum
into guinea-pigs, the latter died in one-quarter of the time that it
usually took them to succumb to a similar dose of an ordinary culture of
the pneumococcus.

It is therefore evident that just as "no chain is stronger than its
weakest link," so in the broad sense no community is stronger than its
weakest group of individuals, and pneumonia, like other epidemics, may
be well described as the vengeance which the "submerged tenth" may wreak
from time to time upon their more fortunate brethren.

Now that we know that under decent and civilized conditions of light and
ventilation the pneumococcus will live but an hour to an hour and a
half, this reduces the risk of direct infection under these conditions
to a minimum. It is obvious that the principal factors in the control of
the disease are those which tend to build up the vigor and resisting
power of all possible victims. The more broadly we study the disease the
more clearly do the data point in this direction.

First of all, is the vivid and striking contrast between hospital
statistics and those gathered from private practice. While many
individuals of a fair wage-earner's income and good bodily vigor are
treated in our hospitals, yet the vast majority of hospital patients are
technically known as the "hospital classes," apt to be both underfed,
overworked, and overcrowded. On the other hand, while a great many both
of the very poor and even of the destitute are treated in private
practice, yet the majority of such cases who feel "able to afford a
doctor," as they say, are among the comparatively vigorous, well-fed,
and well-housed section of the community. And the difference between the
death-rate of the two classes in pneumonia is most significant. In
private practice, while epidemics differ in virulence, the rate ranges
all the way from five per cent to fifteen per cent, the average being
not much in e